Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with high-energy ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses and energies is referred to as an ion implantation recipe.
In conventional ion implantation, ions are extracted from a plasma source and are typically filtered (e.g., for mass, charge, energy, etc.), accelerated and/or decelerated, and collimated through several electro-static/dynamic lenses before being directed to a substrate. FIG. 1 depicts a conventional ion implanter system 100. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components may filter and focus the ion beam 10 before steering it towards a target wafer 120 (located in a wafer plane 12).
A number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 124, and a setup Faraday cup 122, may be used to monitor and control the ion beam conditions. Specifically, measurement of ion dose rate in the ion implantation system 100 may be accomplished using these one or more measurement devices. Because incident ion flux may be measured as an electrical current, the ion dose rate of the target wafer 120 may be calculated by dose count electronics (DCE) (not shown) by taking a measured electrical current and dividing by an aperture area of the one or more measurement devices.
In the design and operation of an ion implanter, ion dose uniformity and ion beam utilization are major concerns since they directly impact the productivity of the ion implanter.
To achieve a uniform distribution of dopants, an ion beam is typically moved across the surface of a target wafer during an implantation process. FIG. 2A shows a typical setup for continuous implantation with an ion beam. In an ion implanter system, e.g., a scanned beam implanter, an ion beam spot 202 may be swept horizontally (i.e., in the X direction) along a scan path 204 across the surface of a wafer 206. A dose control Faraday cup 210 may be used to measure ion beam current. At the same time, the wafer 206 may be translated vertically along a path 208 (i.e., in the Y direction) through a process chamber. Thus, the ion beam spot 202 is scanned with respect to the wafer 206 in both the X and Y directions. The net effect of the movement of the ion beam spot 202 in the X and Y directions is a beam path 20, as depicted in FIG. 2B, that zigzags across the entire surface of the wafer 206 as well as its surrounding area. Since the ion beam spot 202 moves completely off the wafer 206 in each sweep, the total area covered by the ion beam spot 202 may be approximated with a box 22, which may be substantially larger than the wafer 206.
However, the traditional implantation method as illustrated in FIGS. 2A and 2B has a number of problems. For example, such a method often assumes that the ion beam spot 202 maintains the same profile and delivers the same dose at any location. Because conventional measurement devices, e.g., the dose control Faraday cup 118, the traveling Faraday cup 124, and the setup Faraday cup 122, are either intercepting or situated at the side of the wafer 206, the actual ion beam current, and therefore ion dose uniformity, at the wafer 206 may not be accurately measured or determined.
Furthermore, secondary electrons are typically produced upon energetic ion bombardment on these measurement devices. If secondary electrons are not suppressed or confined, most of these electrons may end up colliding with other components of the system 100, which may cause sputtering or heating up of these components or may interfere with the accuracy of ion beam current measurements. Consequently, the accuracy of measuring implant dose is greatly affected by unconfined secondary electrons.
Additionally, in the traditional method, the ion beam spot 202, in its scan path 204, may go completely off the wafer edge in each sweep, which is known as a “full overscan.” Full overscans are deemed necessary to provide a uniform ion dose even at the edges of the wafer 206 and to allow real-time monitoring of ion beam conditions at measurement devices. If the spot size is small, the ion beam is off the wafer surface only briefly. However, if the spot size is large (e.g., greater than about a quarter of the wafer size), as is often the case for low-energy ion beams, the ion beam spot spends almost as much, if not more, time off the wafer as it does on the wafer. As a result, beam utilization becomes extremely low for a low-energy ion beam that is scanned fully off the wafer.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion implantation technologies.